1. Field of the Invention
The present invention relates to the process of photolithography used in fabricating semiconductor devices and the like. More specifically, the present invention relates to a mask used to carry out a photolithographic process and to a method for forming the same.
2. Description of the Related Art
Photolithography is a known process essential to the fabricating of memory devices such as DRAMs, SRAMs and flash memory devices. Photolithography comprises pattern transcription from mask to resist. More specifically, the pattern transcription is carried out by arranging a mask over a resist layer on a wafer, and exposing the resist to light of a specific wavelength through the mask. As a result, the pattern of the mask is transferred to the resist. Ideally, the pattern of the mask is transcribed accurately onto the resist.
However, photolithography is prone to pattern transference problems referred to as “optical proximity effects.” Optical proximity effects occur when forming the very fine patterns necessary to produce a highly integrated circuit. The light waves passing through the closely spaced pattern features of the mask produce interference, thereby distorting the final transferred pattern. These distortions manifest themselves as variations in the dimensions of the patterned resist or as a rounding of ends of the patterns. The optical proximity effects become more pronounced the finer the pattern features are and the thicker the resist is.
When optical proximity effects occur, the dimensions of dense and fine patterns transferred to the wafer are different from each other even though the dimensions of the corresponding features of the mask pattern are the same. In some instances, pattern features are not transferred at all from the mask to the wafer. For example, as shown in FIG. 1, feature 10 at the edge of the pattern is smaller than the same features of the main part of the pattern. As illustrated in FIG. 2, the intensity of the exposure light at the edge α of the pattern is lower than that of the light used to expose the main part of the pattern adjacent the edge α.
A number of techniques have been studied in an attempt to overcome the significant problems of optical proximity effects.
One approach, as is illustrated in FIG. 3, is to design the mask so that the feature 20 at the edge of the pattern is enlarged by an amount considered beforehand, whereby the features of the resultant pattern all have approximately the same dimensions. However, this approach can still be subject to problems resulting from interference such that incomplete patterns are formed.
Another approach, shown in FIG. 4, is to form a line and space type of auxiliary pattern 30 around a main pattern (desired pattern) on the mask. As is illustrated in FIG. 5, the intensity of light transmitted through the region E of the mask containing the auxiliary pattern region is lower than that of light transmitted through the region M containing the main pattern. Specifically, the intensity of the light transmittable through the auxiliary pattern region E is less than the critical intensity (0.20 in this example) required for the patterning of the resist, i.e., required for facilitating the photochemical reaction in the resist.
However, referring to FIG. 6, the change in light intensity at the edge II of the pattern is larger than change in light intensity at the main part I of the pattern. This means that the focus margin at the edge part II is lower than the focus margin at the main part I.
Referring to FIG. 7, complete patterning is obtained in the main part of the pattern within a range of focus of −0.1 to +0.3 μm, and analogous patterning is obtained outside this range of focus. However, complete patterning is obtained in the edge part of the pattern only within a range of focus of from 0.0 to +0.1 μm, and incomplete patterning occurs outside this range of focus of 0.0 to +0.1 μm. Thus, securing a range of focus acceptable for all of the photolithographic processes necessary to fabricate a semiconductor device requires revising a mask having a line and space pattern. That is, a number of such masks are required, which incurs increased manufacturing costs.